Method for homogeneously magnetizing an exchange-coupled layer system of a digital magnetic memory location device

ABSTRACT

A method and apparatus for homogeneously magnetizing an exchange-coupled layer system of a digital magnetic memory cell device comprising an AAF layer system and an antiferromagnetic layer that exchange-couples a layer of the AAF layer system, characterized in that, given a defined direction of magnetization of the antiferromagnetic layer, the magnetic layers of the AAF layer system are saturated in a magnetic field and, afterward, the position of the direction of the antiferromagnetic layer magnetization and the direction of the saturating magnetic field relative to one another is changed, so that they are at an angle α of 0°&lt;α&lt;180° relative to one another, after which the saturating magnetic field is switched off.

BACKGROUND

1. Field of the Invention

The following relates to a method and apparatus for homogeneouslymagnetizing an exchange-coupled layer system of a digital magneticmemory cell device comprising an AAF layer system and anantiferromagnetic layer that exchange-couples a layer of the AAF layersystem.

2. Background Information

A digital magnetic memory cell device stores information on a magneticbasis. An individual memory cell device is generally part of a memorydevice, often also called an MRAM (magnetic random access memory). Sucha memory can carry out read and/or write operations. Each individualmemory cell device comprises a soft-magnetic read and/or write layersystem, separated by means of an intermediate layer from a hard-magneticreference layer system, which is formed as an AAF system in the case ofthe present type of memory cell device. The magnetization of thereference layer of the reference layer system is stable and does notchange in an applied field, while the magnetization of the soft-magneticread and/or write layer system can be switched by means of an appliedfield. The two magnetic layer systems may be magnetized in a parallel orantiparallel fashion with respect to one another. The two aforementionedstates in each case represent a bit of information, i.e. the logic zero(“0”) or one (“1”) state. If the relative orientation of themagnetization of the two layers changes from parallel to antiparallel,or vice versa, then the magnetoresistance changes by a few percent overthis layer structure. This change in the resistance can be used forreading out digital information stored in the memory cell.

The change in the cell resistance can be identified by a voltage change.By way of example, in the case of a voltage increase, the cell may beoccupied by a logic zero (“0”) and, in the event of a voltage decrease,the cell may be occupied by a logic one (“1”). Particularly largechanges in resistance in the region of a few percent have been observedwhen the magnetization orientation changes from parallel toantiparallel, and vice versa, in cell structures of the GMR type (giantmagnetoresistance) or TMR type (tunnel magnetoresistance).

An important advantage of such magnetic memory cells is that theinformation is stored persistently in this way, and is stored withoutmaintaining any basic supply even with the device switched off, and isimmediately available again after the device is switched on, in contrastto known conventional semiconductor memories.

A central component part in this case is the reference layer system,formed as an AAF system (AAF=artificial antiferromagnetic). Such an AAFsystem is advantageous on account of its high magnetic rigidity and therelatively low coupling to the read and/or write layer system by virtueof the so-called orange peel effect and/or by virtue of macroscopicmagnetostatic coupling fields. An AAF system generally comprises a firstmagnetic layer or a magnetic layer system, an antiferromagnetic couplinglayer and a second magnetic layer or a magnetic layer system which iscoupled with its magnetization via the antiferromagnetic coupling layeroppositely to the magnetization of the lower magnetic layer. Such an AAFsystem may be formed e.g. from two magnetic Co layers and anantiferromagnetic coupling layer made of Cu.

In order to improve the rigidity of the AAF system, that is to say itsresistance toward external fields, it is customary to arrange anantiferromagnetic layer at that magnetic layer of the AAF system whichis remote from the read and/or write layer system. Thisantiferromagnetic layer additionally pins the directly adjacent magneticlayer in its magnetization, with the result that the AAF system overallbecomes harder (exchange pinning or exchange biasing).

The magnetic rigidity of the AAF system corresponds to the amplitude ofthe applied external field which is required for rotating themagnetizations of the two ferromagnetic layers in the same direction,that is to say for parallel setting. This limits the magnetic window forread and write applications of such a memory cell device.

In this case it is always an aim for the magnetic layers of the AAFlayer system, which, by way of example, besides the material combinationmentioned in the introduction, may also comprise two ferromagnetic CoFelayers and an Ru layer introduced in between, to be magnetized ashomogeneously as possible, ideally with a single homogeneousmagnetization direction. The magnetization in the case of the memoryelement in question having the AAF layer system and the additionalexchange-coupling or pinning antiferromagnetic coupling layer, e.g. madeof IrMn, is effected in such a way that, after the production of thelayer stack, the layer stack is heated to a temperature greater than theblocking temperature of the antiferromagnetic layer, that is to say e.g.of the IrMn, a strong magnetic field that saturates the two magneticlayers of the AAF layer system being present during this. This leads toan orientation of the magnetic layer magnetizations, and, on account ofthe couplings, also of the magnetization of the antiferromagnetic layer.The temperature is subsequently decreased again. If the external settingmagnetic field is then likewise withdrawn, the magnetization of themagnetic layer which is not coupled to the antiferromagnetic layerbegins to rotate on account of the AAF system coupling.

In this case, however, a multiplicity of so-called 360° walls form inthe magnetization of the layer. These 360° walls, which are manifestedas winding, sinuous lines in the context of carrying out a domainobservation, entail a series of disadvantages. Thus, by way of example,the signal that can be tapped off via the memory element, for example aTMR signal in the case of a TMR memory element (TMR=tunnelmagnetoresistive), is reduced. The magnetization reversal behavior ofthe measurement layer, e.g. made of permalloy, which is separated fromthe AAF layer system by means of a decoupling layer, e.g. made of Al2O3,is also less favorable on account of the leakage fields, those via the360° walls, in which the magnetization rotates once through 360°.

SUMMARY

Embodiments of the invention is thus based on a problem of specifying amethod that enables a homogeneous magnetization, whilst avoiding thedisadvantageous 360° walls to the greatest possible extent.

In the case of a method of the type mentioned in the introduction, it isprovided that, given a defined direction of magnetization of theantiferromagnetic layer, the magnetic layers of the AAF layer system aresaturated in a magnetic field and, afterward, the position of thedirection of the magnetization of the antiferromagnetic layer and thedirection of the saturating magnetic field relative to one another ischanged, so that they are at an angle a of 0°<a<180° relative to oneanother, after which the saturating magnetic field is switched off.

A method is disclosed for homogeneously magnetizing an exchange-coupledlayer system of a digital magnetic memory cell device comprising an AAFlayer system and an antiferromagnetic layer that exchange-couples alayer of the AAF layer system. The magnetic layers of the AAF layersystem are saturated in a magnetic field, given a defined direction ofmagnetization of the antiferromagnetic layer. The position of thedirection of the antiferromagnetic layer magnetization and the directionof the saturating magnetic field is changed relative to one another,such that they are at an angle α of 0°<α<180° relative to one another.After which, the saturating magnetic field is switched off.

A magnetoresistive memory cell device is also disclosed, having ahomogeneously magnetized exchange-coupled layer system comprising an AAFlayer system and an antiferromagnetic layer that exchange-couples alayer of the AAF layer system. The device is formed by saturating themagnetic layers of the AAF layer system in a magnetic field as describedabove.

A magnetoresistive memory device also is disclosed, comprising aplurality of memory cell devices as described above.

Additionally, an apparatus is disclosed for performing a method forhomogeneously magnetizing an exchange-coupled layer system of a digitalmagnetic memory cell device comprising an AAF layer system and anantiferromagnetic layer that exchange-couples a layer of the AAF layersystem. The apparatus comprises a receptacle for a substrate having atleast one memory cell device, and a magnetic field generating device,characterized in that the receptacle and the magnetic field generatingdevice can be rotated with respect to one another.

Further advantages, features and details of the invention emerge fromthe exemplary embodiments described below and also on the basis of thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a memory cell device according to anembodiment of the invention during magnetization, with a temperaturelying above the blocking temperature in the presence of a saturatingmagnetic field.

FIG. 2 illustrates the memory cell device from FIG. 1, after a rotationwith a stationary saturating magnetic field and with the temperaturepreviously having been decreased.

FIG. 3 illustrates the memory cell device from FIG. 2 after the magneticfield has been switched off.

FIG. 4 is a schematic diagram of a first embodiment of an apparatusaccording to the invention.

FIG. 5 is a schematic diagram of a second embodiment of an apparatusaccording to the invention.

DETAILED DESCRIPTION

In the figures below, identical reference symbols designate the sameelements and structures, and descriptions in respect thereof are notrepeated for every occurrence in all the figures. A list of referencesymbols utilized consistently in the figures is provided below:

LIST OF REFERENCE SYMBOLS

-   1 Memory cell device-   2 Reference layer system-   3 Decoupling layer-   4 Soft-magnetic measurement layer system-   5 a, 5 b Word and bit lines-   6 AAF layer system-   7 Lower ferromagnetic layer-   8 Upper ferromagnetic layer-   9 Coupling layer-   10 Antiferromagnetic layer-   11 Apparatus-   12 Receptacle-   13 Rotary table-   14 Substrate-   15 Magnetic field generating device-   16 Apparatus-   17 Receptacle-   18 Substrate-   19 Magnetic field generating device-   20 Rotary device-   T Temperature-   H Magnetic field-   P Arrow-   N Pole-   S Pole

FIG. 1 shows a memory cell device 1 according to an embodiment of theinvention. This memory cell device comprises a so-called reference layersystem 2, which is decoupled from a soft-magnetic measurement layersystem 4 by means of a decoupling layer 3, e.g. Al2O3. The illustrationfurthermore shows the word and bit lines 5 a, 5 b, which run above andbelow at right angles to one another. The reference layer system 2comprises an AAF layer system 6, comprising a lower ferromagnetic layer7, an upper ferromagnetic layer 8 and an antiparallel-coupling couplinglayer 9 arranged between the latter. The ferromagnetic layers may bemade e.g. of Co or CoFe, and the antiparallel-coupling coupling layermay be made of Cu or of Ru. The construction of such an AAF layer systemis sufficiently known.

The reference layer system 2 furthermore comprises an antiferromagneticlayer 10, e.g. made of Ni, FeMn, IrMn, NiMn, PtMn, CrPtMn, RhMn or PdMn,provided below the lower ferromagnetic layer 7. The antiferromagneticlayer 10 couples the magnetization of the lower ferromagnetic layer 7situated above it, i.e. the latter is oriented parallel to the magneticmoments of the antiferromagnetic layer in the interface region. As aresult of this, the magnetization of the ferromagnetic layer 7 is pinnedby exchange biasing. This function and this construction are alsosufficiently known.

For the purpose of setting the magnetization of the magnetic layers 7and 8 and also the magnetization of the antiferromagnetic layer 10, thememory cell device, which is shown as a detail here only in the form ofa schematic diagram and is normally formed with a multiplicity offurther memory elements on a large-area wafer, is heated to atemperature T above the blocking temperature Tblocking of theantiferromagnetic layer 10. At a temperature above the blockingtemperature, the antiferromagnetic layer loses its antiferromagneticproperties. The magnetic moments of the layer can be oriented by anexternal magnetic field. Such an external magnetic field H is applied,the latter being greater than the saturation magnetic field HS requiredto saturate the magnetizations of the magnetic layers 7, 8. Themagnetizations of the layers 7, 8, represented by the long arrows, areclearly oriented parallel to the external field, and the magneticmoments of the antiferromagnetic layer 10 are also orientedcorrespondingly, the moments near the interface, which lie in theinterface region with respect to the magnetic layer 7, coupling parallelthereto.

The temperature is subsequently decreased, so that it is below theblocking temperature Tblocking. When the temperature falls below theblocking temperature, the antiferromagnetic layer 10 undergoestransition to the antiferromagnetic state again; the magnetization is“frozen”. However, the external magnetic field is still present insaturating fashion.

As shown in FIG. 2, a rotation through an angle a is subsequentlyeffected, when a is 90° in this case. The magnetization of theantiferromagnetic layer 10 clearly remains in the formerly setdirection, i.e. it does not change despite the external saturatingmagnetic field H that is still present. The angle a is the angle whichthe magnetization direction of the layer 10 assumes with respect to theexternal magnetic field H, as is depicted in FIG. 2.

In contrast to the stationary magnetization of the antiferromagneticlayer 10, in the event of a rotation of the memory cell device withrespect to the stationary external magnetic field, the magnetizations ofthe magnetic layers 7 and 8 rotate during the rotation, i.e. they stillremain parallel to the external magnetic field H, as is clearly shown inFIG. 2. They are thus at an angle with respect to the direction ofmagnetization of the antiferromagnetic layer 10, the angle likewisebeing a=90°.

Afterward, see FIG. 3, the external magnetic field is switched off. Thishas the effect that differently directed rotation processes take placein the two magnetic layers 7 and 8. While the magnetization of the lowermagnetic layer 7, which lies adjacent to the antiferromagnetic layer 10,in the example shown, is established toward the right and thus parallelto the moments of the antiferromagnetic layer 10 which are near theinterface, which is governed by the strong exchange coupling between thetwo layers, the magnetization of the magnetic layer 8 rotates in theopposite direction on account of the coupling properties of the couplinglayer 9. FIG. 3 shows, by means of broken lines, in each case theinitial positions of the magnetizations, as are known from FIG. 2, andthe solid arrows represent the respective end position after rotationhas taken place. Clearly, both magnetizations rotate only through arelatively small angle, namely through 90°, governed by the previouslyadopted setting of the memory cell device with respect to the externalmagnetic field, as described with reference to FIG. 2. There exists forthe respective magnetization of the layers 7, 8 an excellent shortestrotation path in order to rotate into the setting respectively definedin a coupling-dictated manner. A decomposition of the magnetization intodifferent magnetization regions or the formation of undesirable 360°walls is precluded on account of this preferred direction of rotation.

FIG. 4 shows an apparatus 11 of a first embodiment, which serves forcarrying out the method described with reference to FIGS. 1–3. Itcomprises a receptacle 12 in the form of a rotary table 13, which can berotated as shown by the arrow P. A substrate 14, at which are formed amultiplicity of memory cell devices (which form a memory device) withthe layer system described in the introduction, is arranged on saidrotary table. The rotary table 13 itself can expediently be heated. Itis assigned a magnetic field generating device 15, as is indicated bythe two magnetic poles N and S. In order to carry out the method, thesubstrate 14 is positioned on the rotary table 13, followed by the highdegree of heating and the application of the external saturatingmagnetic field, after which the cooling and subsequent rotation andfinally the switching off of the external magnetic field are performed,as described in detail with reference to FIGS. 1–3.

FIG. 5 shows a further apparatus 16 according to an embodiment of theinvention. In the case of this apparatus, the receptacle 17 is formed asa heatable table which, however, cannot be rotated. A substrate 18 isalso to be arranged on the receptacle 17. By contrast, the magneticfield generating device 19, which is again indicated by the two poles Nand S in this case as well, is mounted in a rotatable manner on asuitable rotary device 20, e.g. in the form of a rotary table, as islikewise shown by the arrow P. A somewhat modified method for producinga homogeneous magnetization is possible in the case of this apparatus.Instead of the memory cell device being rotated with respect to thestationary magnetic field in the manner described with reference to FIG.2, here the memory cell device remains stationary, while the magneticfield is rotated for the purpose of setting the angle ‘a.’

Embodiments of the invention bring the magnetization of theantiferromagnetic pin layer, which, on account of the very strongexchange coupling with respect to the adjoining magnetic layer of theAAF layer system, retains the magnetization thereof, and the directionof the magnetic field at an angle<180° and >0°, and subsequentlyswitching off the magnetic field. Since the magnetization direction ofthe antiferromagnetic layer has already been established, themagnetization of the adjoining magnetic layer of the AAF system willthen automatically rotate in the same direction. On account of thecoupling of the second magnetic layer of the AAF system via the couplinglayer arranged in between, the magnetization of the second magneticlayer then rotates in the opposite direction. On account of the settingof the direction of the antiferromagnetic layer magnetization and of thesaturating magnetic field still present beforehand with respect to oneanother, however, rather than a rotation through 180°, it is nownecessary only for a rotation through a significantly smaller angle totake place. This is because, on account of the angular setting and thesaturating magnetic field still present beforehand, the still saturatedmagnetization of the two magnetic AAF layers is not at an angle of 0 or180° with respect to the magnetization of the antiferromagnetic layer,but rather at an intermediate angle. It is only through thisintermediate angle that the magnetization of the second magnetic AAFlayer now has to be rotated. The magnetization will thus preferablyrotate exclusively in this one direction which has the shorter rotationpath and consequently permits a more favorable rotation in terms ofenergy. This therefore precludes decomposition of the magnetization orthe formation of the 360° walls, which primarily form when themagnetization has to rotate through 180°, since in this case a rotationis possible in two directions, namely from 180° to 0° and from 180° to360° (ultimately to the “left” and “right”), which leads to the wallformation. The setting of the magnetization directions thus defines apreferred direction of rotation in which the layer magnetizationrotates, which advantageously leads to the 360° walls being avoided.

An angle ‘a’ of 60° to 120°, in particular of 90°, can expediently beset. In the case of an angle of 90°, the magnetization of the adjacentAAF layer that is pinned by means of the antiferromagnetic layer has torotate through 90° and the magnetization of the second magnetic layeralso has to rotate likewise through 90°, but in the other direction. Thepath is relatively short overall for both magnetizations, with theresult that a homogeneous magnetization is established.

For the purpose of setting the angle, a number of possibilities areconceivable in the context of the magnetization. On the one hand, thememory element may be rotated with respect to the stationary magneticfield; as an alternative to this, it is possible to move the magneticfield with respect to the stationary memory element. Finally, it is alsopossible for both to be moved with respect to one another.

Methods according to embodiments of the invention make it possible toavoid the formation of 360° walls even during the first magnetization ofa memory element. For the purpose of setting the magnetization of theantiferromagnetic layer, the temperature is increased above the blockingtemperature of the layer, the saturating magnetic field being presentduring the increase in temperature, after which the temperature isdecreased and the magnetization of the layer system is set. Thus, inthis case, the saturation of the magnetic layers of the AAF system andthe setting of the magnetization of the antiferromagnetic layer areperformed simultaneously, which, since the temperature lies above theblocking temperature, is adapted to the magnetization direction of theadjacent magnetic layer of the AAF system. The temperature issubsequently decreased, with the result that the magnetization of theantiferromagnetic layer is as it were frozen after the temperature hasfallen below the blocking temperature. The memory element and/or themagnetic field is subsequently rotated for the purpose of setting theangle a with the magnetic field still present, after which this isswitched off and the magnetizations of the two magnetic layers of theAAF system rotate in the two different directions.

However, methods according to the invention equally also make itpossible to subsequently homogenize a memory element which has alreadybeen magnetized and has 360° walls, that is to say is magnetizedinhomogeneously. For this purpose, the magnetization of the AAF layersystem is saturated in a sufficiently high magnetic field without thetemperature being increased beforehand. After saturation, the system isrotated with respect to the magnetic field, e.g. through 90° withrespect to the original saturation direction of the antiferromagneticlayer, in which case the latter has expediently been oriented parallelto the external saturation magnetic field beforehand or the memoryelement has been positioned correspondingly. After rotation, theexternal magnetic field is driven back, in such a way as to rotate backthrough the corresponding angular sections the layer magnetizationswhich lay in the direction of the external magnetic field despite therotation on account of the saturation.

In addition to such methods, a magnetoresistive memory cell device isdisclosed that has been magnetized in accordance with the method.Furthermore, a magnetoresistive memory device is disclosed thatcomprises a plurality of memory cell devices.

Furthermore, the invention relates to an apparatus for carrying out themethod, comprising a receptacle for a substrate having at least onememory element, and also a magnetic field generating device. Theapparatus is distinguished by the fact that the receptacle and themagnetic field generating device can be rotated with respect to oneanother. According to a first refinement of the invention, thereceptacle may be a rotary table and the magnetic field generatingdevice may be stationary, i.e. the substrate, at which, by its nature, amultiplicity of memory elements are formed, is in this case rotated withrespect to the stationary magnetic field. As an alternative, thereceptacle may be stationary and the magnetic field generating devicemay be rotatable.

It is particularly advantageous if the receptacle, and if appropriatethe rotary table, can be heated in order to heat the substrate asrapidly as possible to a temperature above the blocking temperature, ifthis is required.

Overall, the invention proposes a method which is simple to practice,requires no additional time-consuming or costly method steps and enablesthe homogeneous magnetization of the relevant layers of a memory celldevice or memory system.

The foregoing disclosure of embodiments of the present invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Many variations and modifications of the embodimentsdescribed herein will be obvious to one of ordinary skill in the art inlight of the above disclosure. The scope of the invention is to bedefined only by the claims appended hereto, and by their equivalents.

1. A method for homogeneously magnetizing an exchange-coupled layersystem of a digital magnetic memory cell device comprising an AAF layersystem and an antiferromagnetic layer that exchange-couples a layer ofthe AAF layer system, comprising: saturating the magnetic layers of theAAF layer system in a magnetic field, given a defined direction ofmagnetization of the antiferromagnetic layer; changing the position ofthe direction of the antiferromagnetic layer magnetization and thedirection of the saturating magnetic field relative to one another, suchthat they are at an angle α of 0°<α<180° relative to one another; andswitching off the saturating magnetic field.
 2. The method as in claim1, further comprising the step of setting an angle α of between 60° and120°, in particular of 90°.
 3. The method as in claim 1, furthercomprising the step of rotating the memory cell device with respect tothe stationary magnetic field for the purpose of setting the angle. 4.The method as in claim 1, further comprising the step of moving themagnetic field with respect to the stationary memory cell device for thepurpose of setting the angle.
 5. The method as in claim 1, furthercomprising the step of moving the magnetic field and the memory celldevice for the purpose of setting the angle.
 6. The method as claim 1,further comprising the steps of: increasing the temperature above theblocking temperature of the layer for the purpose of setting themagnetization of the antiferromagnetic layer, the saturating magneticfield being present during the increase in temperature, and after whichthe temperature is decreased and the magnetization of the layer systemis set.
 7. A magnetoresistive memory cell device, having a homogeneouslymagnetized exchange-coupled layer system comprising an AAF layer systemand an antiferromagnetic layer that exchange-couples a layer of the AAFlayer system, formed by saturating the magnetic layers of the AAF layersystem in a magnetic field, given a defined direction of magnetizationof the antiferromagnetic layer, changing the position of the directionof the antiferromagnetic layer magnetization and the direction of thesaturating magnetic field relative to one another, such that they are atan angle α of 0°<α<180° relative to one another, and switching off thesaturating magnetic field.
 8. A magnetoresistive memory devicecomprising a plurality of memory cell devices, wherein the memory celldevices have a homogeneously magnetized exchange-coupled layer systemcomprising an AAF layer system and an antiferromagnetic layer thatexchange-couples a layer of the AAF layer system, formed by saturatingthe magnetic layers of the AAF layer system in a magnetic field, given adefined direction of magnetization of the antiferromagnetic layer,changing the position of the direction of the antiferromagnetic layermagnetization and the direction of the saturating magnetic fieldrelative to one another, such that they are at an angle α of 0°<α<180°relative to one another, and switching off the saturating magneticfield.
 9. An apparatus for performing a method for homogeneouslymagnetizing an exchange-coupled layer system of a digital magneticmemory cell device comprising an AAF layer system and anantiferromagnetic layer that exchange-couples a layer of the AAF layersystem, comprising: a receptacle for a substrate having at least onememory cell device, and a magnetic field generating device,characterized in that the receptacle and the magnetic field generatingdevice can be rotated with respect to one another.
 10. The apparatus inclaim 9, wherein the receptacle is a rotary table and the magnetic fieldgenerating device is stationary.
 11. The apparatus in claim 9, whereinthe receptacle is stationary and the magnetic field device is rotatable.12. The apparatus in claim 9, wherein the receptacle, and if appropriatethe rotary table, can be heated.
 13. A method for homogeneouslymagnetizing an exchange-coupled layer system of a digital magneticmemory cell device comprising an AAF layer system and anantiferromagnetic layer that exchange-couples a layer of the AAF layersystem, comprising: saturating magnetic layeres of the AAF layer systemin a magnetic field, given a defined direction of magnetization of theantiferromagnetic layer; changing by rotation the position of thedirection of the antiferromagnetic layer magnetization and direction ofthe saturating magnetic field relative to one another, so that they areat an angle a of 0°<a<180° relative to one another, and after which,switching off the saturating magnetic field.